1. Field of the Invention
The present invention relates to a disk drive system in which the actuator seek time access is improved. More particularly, the present invention relates to a disk drive system in which the seek time access is decreased while utilizing the same power supply for powering the movement of the actuator.
2. Art Background
Disk drives are popular media for storing data in machine readable form. There is an ever-present desire to decrease the average time required to access any desired data on the disk ("the seek time") while maintaining reasonable costs for performing the access.
A simplified block diagram of a disk drive system is shown in FIG. 1. The disk 10 rotates about a spindle 15, driven by a spindle motor 20, as controlled by the spindle motor control 25. The speed of revolution of the disk is therefore controlled by the spindle motor 20 and control 25 system. The head assembly 30 is attached to an actuator 35, controlled by an actuator motor 40 and actuator control 45, to move the actuator 35 and therefore the head assembly 30, to access different tracks of the disk 10.
Disk drive interface controller 50 controls the overall operation of the disk drive system and the exchange of data between the disk drive and the host device such as a computer system or CPU. Read/write control 55 controls the read and write operations on the disk by the head assembly 30. Power supply 60 provides the necessary power to drive the disk drive system, including spindle motor 20 and actuator motor 40.
Typically, in a disk drive system, the current to the actuator motor is controlled in one of two ways: voltage mode or current mode. In a voltage mode, seeks a voltage at or near the full voltage available at the power supply is provided to the actuator motor in both the accelerate and decelerate portions of the seek in order to reach a specified track on the disk drive. If the distance the actuator is to move to the next location is great, resulting in a long seek, the actuator may go into a constant velocity or coast state. The coast state typically corresponds to the maximum velocity the actuator can physically move while maintaining control of actuator movement. Thus, the seek consists of, in general, an accelerate portion, a coast portion at a constant velocity and a deceleration portion, resulting in the movement of the head assembly to a track on which a read or write operation is to be performed. An exemplary voltage mode current profile for the actuator is shown in FIG. 2a. During both the accelerate and decelerate phases of the seek, the maximum voltage is put across the actuator motor, thus producing the maximum current. The slope on the current profile is due to motor back EMF.
Although a voltage mode seek enables the fastest possible seek time, the. hardware and software cost needed to implement the portions of the seek, for example, the deceleration trajectory following is significant as a deceleration profile must be continuously calculated according to the current input to the motor. In particular, the way an actuator moves to the proper track during a seek is to try to follow a velocity versus position schedule that is created with the knowledge of what the deceleration of the actuator is. Since a voltage mode seek puts all of the available voltage across the actuator, and the voltage supply will vary with many factors (time, the system the drive is installed in, etc.) the only way to know the deceleration attainable by a drive would be to monitor the supply voltage. Furthermore, exact torque of the actuator, inertia, external forces (flex lead bias, windage, etc.) would also have to be known. After all of these parameters were identified, the velocity versus position schedule is created using the equation velocity =square.sub.-- root (2*deceleration*position).
There are a number of practical problems with the implementation. One problem is that all of the parameters that affect deceleration cannot be exactly identified. The other problem is that calculating the square root function with the processors used in disk drives is not feasible as the processors are not powerful enough to timely perform the calculations. Furthermore, the peak current drawn during the deceleration portion of the seek is significantly more than the average value. This is a result of the back EMF of the actuator effectively adding to the supply voltage so that more current is drawn from the power supply. Conversely, during the accelerate portion of the seek, the back EMF is subtracting from the supply voltage, thereby causing the current to become lower as the actuator moves faster. Back EMF (ElectroMotive Force) is a property of voice coil actuators in which a voltage will be produced in the motor that is proportionate to the velocity of the motor. During the acceleration this voltage reduces the available voltage and current as the actuator velocity increases, and during the deceleration it adds to the voltage across the motor thus increasing the current available. In some systems, derating by this extra voltage provided more margin in the design. In the push for faster seek times, some systems use this extra voltage in both the accelerate and the decelerate portions of the seek. This means that the velocity versus position scheduler is now made assuming the deceleration will change with the velocity of the actuator. Although this gives the system the ability to decelerate faster and thus improve seek times, the peak current drawn increases.
Therefore, in summary, the voltage mode is an idealized solution utilizing full acceleration and deceleration. The drawback is that actuator response varies from actuator to actuator, for example, having different torques. Furthermore, different power supplies may provide different responses.
The second method used to move the actuator is a current mode seek. In a current mode seek, the minimum current that can be put into the actuator is calculated for both the accelerate and decelerate portions of the seek and these values are used throughout the seek.
Typically, these values are calculated in accordance with an estimate of the performance of a worst case actuator (e.g., an actuator with the lowest torque), thereby insuring that all actuators be able to follow the worst case acceleration/deceleration schedule. Therefore, each actuator configured to operate using current mode seek operates in the worst case scenario, decreasing the potential performance of those actuators which do not operate in accordance with the worst case. Furthermore, the current mode is slow, as a minimum value is input to drive the actuator during the accelerate and decelerate portions, resulting in a longer seek time.
In practice, it has been found that the current mode is easier to implement and the peak current used, and therefore the drain on the power supply, is less. FIG. 2b is an illustration of the current mode current profile for an actuator. During the accelerate portion of a seek, a constant current value is provided to the actuator. Similarly, during the decelerate portion, a constant current input is provided to drive the actuator.
As mentioned earlier, the spindle motor drives the spindle and therefore the revolution of the disk. The spindle motor also utilizes the power supply. FIG. 2c is an illustration of the current profile for the current driving the spindle motor. The spindle motor is considered independent from the actuator motor. The spindle motor current is held preferably at a time average value such that the motor maintains a velocity within a necessary tolerance.